If you're writing an essay, there are certain structures and guideline requirements to follow. If you're writing a reflection, the only guide is your emotions and your memories, and the only requirement is to be honest about what you think and how you feel.
An essay is a short, informative piece of writing. It includes an introductory paragraph with a clear thesis statement, a body of at least three more paragraphs that relate back to the thesis statement, and a conclusion that sums up the writer's points and makes inferences about them.
The four main types of essays (persuasive, expository, descriptive, and narrative) each have their own guidelines to follow, and it's important to understand what is expected before outlining your essay and trying to organize your thoughts.
If you've been assigned a writing piece that is strictly a piece of reflective writing, you will most likely be given guidelines to follow that were chosen by the teacher or professor giving the assignment. However, there are no strict guidelines to follow in writing a traditional reflective piece and you don't have to wait for one to be assigned to practice and benefit from them. Think of it as being similar to writing in your diary or journal but with an attempt to explore why the experience you're writing made you feel the way you did and why you reacted to it in the way you did.
Reflective writing helps to capture your thoughts and feelings on an event, a current news story, a memory, or any other experience. Reflections can help you discover lessons you learned from that experience, develop your writing skills, and make sense of things that happen around you. The only real requirement is that you explore your own mind through your writing. You won't be just recounting a story or summarizing things that happened, reflection goes deeper into the writer's response to the subject being reflected upon.
The most important thing to remember about a reflection is that there is no correct or incorrect way to respond to any experience, so your reflection is not a judgment of your reactions. It is simply an exploration of them; therefore, exploring why you had that reaction and what past experiences you've had that caused you to react or feel the way you did is key to writing a good reflection.